This wasn’t working prior to 8.1.5 but .strm files managed by Jellyfin won’t playback in Infuse - at least in my experience this is the case, could it possibly be a similar issue which affected Plex?
Would you be able to upload one of the problematic files so we can review it here?
If you can also send in a report from your device (and post the code it gives you here) that could be helpful in understanding what is going on.